With the development of technologies, the operations of many equipment and apparatuses, such as small cells, require the use of an oscillator. However, after these equipment and apparatuses have been used for a period of time, the change of the external environment (e.g., the temperature and the humidity) or the aging of hardware tends to cause an offset of the operation frequency of the oscillator thereof. When the offset of the operation frequency of the oscillator becomes too large, the equipment and apparatuses may not be able to operate normally. For example, when the small cell is synchronous with other apparatuses, the clock precision that needs to be provided by its oscillator is 50˜250 parts per billion (ppb). In this case, if the oscillator of the small cell cannot provide the clock precision ranging between 50 ppb and 250 ppb, some operations cannot be performed normally by the small cell, e.g., the handover operation.
Currently, some conventional technologies are available for synchronization between apparatuses. Most of the conventional technologies are for time synchronization, e.g., the Network Time Protocol (NTP), the Precision Time synchronization Protocol (PTP), and the Global Positioning System (GPS). The Synchronous Ethernet (SyncE) among conventional technologies is for frequency synchronization. Additional hardware needs to be provided when the frequency synchronization is achieved by adopting the PTP and the GPS for calculation or by directly using the SyncE. However, the hardware is very expensive, so it represents a great burden for users. Moreover, the use of the GPS is impossible in indoor environments.
Accordingly, it is important to find a technique to calibrate the offset of the operation frequency at a low cost.